1. Field of the Invention
The invention relates to a method of extracting phase errors caused by the atmosphere in the backscatter signal of a coherent imaging radar system carried by a carrier from radar raw data and an apparatus for carrying out the method.
2. Description of the Prior Art
By means of coherent imaging radar systems, which may be carried by aircraft and spacecraft, to determine the instantaneous signal phase the variation of the flight path can be exactly determined for example with the aid movement sensors. Since however the atmosphere is not homogeneous either in the lower region, the troposphere, or in the upper region, the ionosphere, the radar signal received has additional phase fluctuations. Consequently, imaging radar systems carried by aircraft or spacecraft, particularly those with synthetic aperture, even when they move on undisturbed or exactly known flight paths have a restricted resolution in the flight direction, this being due to the atmospherically induced phase errors or fluctuations.
In coherently operating imaging radar systems these phase errors lead to distortions and errors; this considerably impairs the quality of the radar images; under some circumstances, an imaging even becomes completely impossible. The resolution in the range direction therefore depends on the system bandwidth and is not influenced by the atmosphere. For example, the resolution obtainable at present from spacecraft without phase error correction is about 10 m in the flight direction.
Criteria for the quality of an imaging are derived from the resolution, the image contrast and the geometrical distortion. However, all these criteria are impaired by phase errors.
To obtain high resolution, high contrast and a low geometrical distortion of the imaging, the raw data received must therefore be corrected before the processing or generation of an image. This correction can be carried out in real time or off-line. A real time correction during the reception of the backscatter signal is carried out by digital or analog control elements whilst an off-line correction is carried out on the ground after storing the raw data with the aid of computer programs.